Track gauges for aligning and focussing the imaging system in a high speed document handling system

ABSTRACT

A set of track gauges for use in aligning, focussing and normalizing an imaging system of a high speed document processor have been provided. Each of the gauges fit within a track (14) in front of the imaging system (10 or 12). The depth gauge (70) is used to align the secondary light beam (26) with the primary light beam (22) so that all documents passing through the track (14) will be evenly illuminated from two points symmetrically offset from a line normal to the track (14). In addition, the depth gauge (70) is used to align a normally reflected beam (32) with a photodector array (56). Two different trunnion mirrors (54 and 28) are adjusted in the two different alignment operations. The focussing guage (80) includes a set of black 87 and white 88 strips positioned on a middle region 86. The white reference gauge (90) has a reflective white coating in its middle region 96. An oscilloscope (100, 210, 300, or 400) is used to display the intensity of reflected light (32) impinging on the photodector array (56) duringy all optical alignment and focussing operations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the co-pending U.S. patent applicationentitled "Electronic Image Lift" which was filed on Oct. 10, 1989 andhas Ser. No. 419,574. The above-identified application is assigned to acommon assignee and is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to high speed document handlingsystems and, more specifically, to a method and a set of gauges foraligning and focusing the imaging devices used in a high speed documenthandling system.

2. Description of the Prior Art

High speed document handling systems are used to quickly capture andstore information present on the surfaces of a document. After theinformation has been captured, the information can be processed atremote stations without physically transferring the document. High speeddocument handling systems have been in use in the banking industry toprocess checks, in the credit industry to process credit card receipts,and in wholesale and retail operations to process remittance documents.

Early automated document handling systems required the documents tocontain coded data which could be read by a bar code reader or similardevice. Because many documents are handwritten, document handlingsystems which rely on coded information have limited utility.

Recently, much effort has been made to adapt image capturing andcharacter recognition schemes for use in a high speed document handlingsystem. In operation, a document is imaged using a light source toilluminate the document and a photodetector or camera array to receivelight reflected from the document as the document is moved through atrack past the imaging station at high speed. After the document hasbeen imaged, the signals from the photodetector or camera array can beanalyzed to determine and store the information on the document.

Correct operation of the imaging system is critical to the performanceof any high speed document handling system. Improper alignment of thelight sources and the photodetector array and/or improper focussing ofthe light will reduce the intensity of light received by thephotodetector array and make the information on the documentindistinguishable from background noise. In the past, it has been thepractice to merely drop a piece of white paper within the track of thedocument handling system and to adjust the optics to maximize the peaksignal output from the photodetector array. This method of alignment andfocussing leads to mixed results since there is no control over theprecise placement of the white paper within the track. In onecalibration run, the white paper may be positioned closer to the frontwall of the track while in another calibration run, the white paper maybe positioned closer to the rear wall of the track. In addition, thewhite paper could be positioned at an angle within the track. Withoutknowing the precise position of the white paper in the track, atechnician is liable to set the light output too high such thatinformation received by the photodetector will be lost because ofsaturation of the analog to digital (A/D) converter.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a set of gaugeswhich enable a technician to properly and repeatably align and focus theimaging system of a high speed document handling system.

According to the invention, three gauges are used by the technician foraligning, focusing and normalizing an imaging system in a high speeddocument handling system. In a preferred embodiment, the documenthandling system has two imaging systems which image opposite sides ofthe document as it travels through the track. The imaging systemsoptically interact with the document through window slots in the track.Preferably, the window slots in the front and rear of the track areoffset so that illumination light from the front side of the track doesnot interfere with the light reflected from the back side of a documentto the back side imaging system.

Typically, an imaging system includes three beams of light. Two beams oflight illuminate documents in the track through the window in the trackwall and one beam of light is reflected from the document back throughthe window in the track wall to a photodetector array. The illuminatingbeams are rectangular in shape and may be supplied by an optical fiberarray. The window in the track is rectangular and is large enough toview a portion of the document from its top to its bottom. The two beamsof light used to illuminate the document are symmetrically offset from apath normal to the track. Illumination from two symmetrically offsetpoints is preferred since this arrangement reduces shadowing effectscaused by wrinkles in the document which would mask information on theface of the document.

In order for the imaging system to work correctly, all three beams needto be aligned. Alignment is achieved using a depth gauge. The depthgauge is rectangular in shape where the height and width dimensions arelarge enough to cover the entire window in the track wall. The thicknessof the depth gauge at its top, bottom and side edges is sized to fitsnugly within the track, i.e., the thickness is ideally equivalent tothe distance between the front and rear walls of the track. In themiddle portion of the depth gauge, the thickness is varied in ahorizontal stripe pattern extending up its height to create two sets ofplateau regions. One set of plateau regions has a thickness a fixedamount less than half the thickness of the depth gauge and the other setof plateau regions has a thickness that same fixed amount more than halfthe thickness of the depth gauge. As an illustrative example, the trackis eighty thousandths of an inch (mils) wide; therefore, the top,bottom, and side edges of the depth gauge are 80 mils thick. The firstand second set of plateaus must be at thicknesses equidistant from thecenterpoint of 40 mils; therefore, the first set of plateaus having athickness of 20 mils and the second set of plateaus having a thicknessof 60 mils would be satisfactory. It is anticipated that the sets ofplateau regions could be varied by having the plateaus in one set of theplateau regions have graduated thicknesses less than half the thicknessof the depth gauge where the plateaus in the other set of plateauregions have corresponding graduated thicknesses more than half thethickness of the depth gauge. Each set of plateau regions has a whitereflective coating for reflecting the illuminating light.

Alignment of the three beams is achieved by adjustment of a pair ofmirrors. The depth gauge is placed in the track in front of the windowin the track wall. An oscilloscope is connected to the output of thephotodetector array. A technician makes rotating adjustments, as will bedescribed in more detail below, to the mirrors while watching theoscilloscope display. The alternating plateaus on the depth gauge createregions of imbalance on the oscilloscope output when the illuminatinglight beams are slightly out of alignment. Alignment is achieved when abalanced, maximum output in the center region of the oscilloscope isobserved. Using a gauge with a white surface positioned in the middle ofthe track would not be as sensitive to slight misalignments. In apreferred embodiment, a primary beam, which illuminates the documentfrom a thirty degree angle relative to normal, is fixed, and thesecondary beam, which illuminates the document from a thirty degreeangle symmetrically offset from normal with respect to the primary beam,is adjusted to line up with the primary using a first trunnion mirror.The reflected beam from the depth gauge is aligned with the center ofthe photodetector array using a second trunnion mirror.

Focussing the reflected light beam on the photodetector array isaccomplished using a lens slidably mounted between the trunnion mirrorand the photodetector array. When focussing, the depth gauge is replacedin the track by a focussing gauge. The focussing gauge has the sameheight and width dimensions as the depth gauge. In addition, the top,bottom, and side edges of the focussing gauge have a thicknessequivalent to the distance between the track walls so that it fitssnugly therein. The middle region of the focussing gauge has a thicknessmachined down to half the overall thickness of the focussing gauge. Aseries of black and white stripes are positioned on the focussing gaugeup its height. The black and white stripes may be photographicallyreproduced and attached to the middle region of the focussing gauge,painted thereon, or supplied by some other suitable technique. Inoperation, a technician moves the lens towards and away from thephotodetector array while watching the oscilloscope. Focussing isachieved when a sinusoidal wave having a maximum peak to valleydisplacement is seen on the oscilloscope display. The peaks of thesinusoidal wave represent light reflected from the white stripes and thevalleys represent the light reflected from the black stripes. Thesinusoidal wave is the "beat frequency" between the black and whitestripes on the focussing gauge and the discrete photodetectors on thephotodetector array.

After the imaging system has been aligned and focussed, a whitereference gauge is placed in the track in front of the window in thetrack wall. The white reference gauge has the same height and widthdimensions as the depth and focussing gauges. Also, the white referencegauge has a thickness at its top, bottom and side edges which allows itto fit snugly within the track. The white reference gauge has a middleregion with a thickness machined to a point less than the overallthickness of the gauge. In an exemplary environment where track is 80mils wide, the white reference gauge would be 80 mils wide at its top,bottom, and side edges and the middle region would be machined down0.015 inches to a thickness of 65 mils from the back surface of thegauge. The middle region has a white reflective coating. The whitereflective coating can be provided by white paint, a chemical coating,or a white plastic material being attached to the gauge. In addition,the gauge itself could be constructed of a white plastic material oranodized aluminum, or any other suitable material. The white referencegauge is used to normalize the output from the photodetector array,i.e., the light detected by the photodetector array which is reflectedfrom the white reference gauge represents a maximum amount of lightwhich the imaging system will encounter when processing documents. Theelectronics of the imaging system used for capturing and processing thedocument images are normalized by treating signals received by thephotodetector array which approach those obtained with the whitereference gauge as background.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following description of a preferred embodiment ofthe invention with reference to the drawings, in which:

FIG. 1 is a partial, cut-away isometric view of the track in a highspeed document handling system with FIG. 1A showing an imaging slit in aportion thereof;

FIG. 2 is a representational top view of the front and rear imagingsystems used in the high speed document handling system showing thepaths of the two symmetrically offset illuminating beams and part of thepath of the normally reflected beam;

FIG. 3 is a schematic diagram showing the path of the normally reflectedbeam towards the photodetector array;

FIGS. 4a-b are front and cut-away side views, respectively, of the depthgauge;

FIGS. 5a-b are front and cut-away side views, respectively, of thefocussing gauge;

FIGS. 6a-b are front and cut-away side views, respectively, of the whitereference gauge;

FIGS. 7a-c are representational side views of the path of a reflectedbeam when the depth gauge is placed in the track and the primary beam isbeing aligned with the reflected beam;

FIGS. 8a-c are front views of oscilloscope displays which correspond tothe output from the photodetector arrays shown in FIGS. 7a-7c,respectively;

FIGS. 9a-9c are representational top views of the path of lighttravelled when the secondary beam is being aligned with the primarybeam;

FIGS. 10a-10c are representational, partial side views of the alignmentsillustrated in FIGS. 9a-9c, respectively;

FIGS. 11a-11c are front views of oscilloscope displays which correspondto the output from the photodetector array as the secondary beam isaligned with the primary beam as shown in FIGS. 9a-9c, respectively;

FIGS. 12a-12c are schematic diagrams showing the movement of thefocusing lens for focusing the reflected light onto the photodetectorarray;

FIGS. 13a-13c are front views of the oscilloscope displays whichcorrespond to the output from the photodetector array as the focusinglens is moved in the manner shown in FIGS. 12a-12c, respectively; and

FIGS. 14a-14b are front views of oscilloscope displays which correspondto favorable and unfavorable output from the photodetector array whenthe white reference gauge is placed in the track.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown front and back imaging systems 10 and 12, respectively, positionedto illuminate and capture the image of documents (not shown) whichtravel through the track 14 at high speed. The track 14 is comprised ofside walls 16 and 18 which are spaced approximately 80 mils apart Eachside wall 16 and 18 includes a rectangular slit or window (see frontslit S_(F) FIGS. 1A, 2, rear slit S_(R), FIG. 2) extending linearly fromthe top of the track 14 to the bottom of the track 14 through which adocument in the track 14 can be imaged. The imaging systems 10 and 12are linearly offset (shown more clearly in FIG. 2) along the length ofthe track 14 so that illumination light from one imaging system 10 or 12does not interfere with reflected light which is to be detected by theopposite imaging system 10 or 12, i.e., the offset arrangement avoids aback lighting effect.

FIG. 2 shows a representational top view of the illumination part of theimaging systems 10 and 12, where each imaging system 10 and 12 iscomprised of a fiber optic bundle 20 or 20' for projecting a primarylight beam 22 or 22' toward the track 14, a beam splitter 24 or 24'which permits half of the primary light beam 22 or 22' to passtherethrough and reflects the other half (now called secondary lightbeam 26 or 26') to a trunnion mirror 28 or 28' which reflects thesecondary light beam 26 or 26' towards the track 14, and a detectingsystem, shown generally as arrows 30 or 30' (described in more detail inconnection with FIG. 3), which detects normally reflected light 32 or32'. The fiber optic bundle 20 or 20' is comprised of a stack of opticalfibers which produce a linear bar of light (FIG. 2 only showing the topfiber 34 or 34' and the linear bar extending down into the paper) Thelinear bar forming primary light beam 22 is tall enough to illuminate adocument sent through track wall 16, via front slit S_(F) from its topto its bottom (likewise beam 22' through rear track wall 18, via rearslit S_(R), offset from S_(F)). The primary light beam 22 or 22' whichpasses through beamsplitter 24 or 24' is focussed by condensing lens 36or 36' for optimum illumination at the center line 38 of the track 14.The secondary light beam 26 or 26' reflected by beam splitter 24 or 24'is focussed for optimum illumination at the center line 38 of the track14 by condensing lens 40 or 40'. Positioning the condensing lens 40 or40' in the path of the secondary light 26 or 26' before the trunnionmirror 28 or 28' provides some advantages in the physical size of theimaging system 10 or 12.

This invention is particularly concerned with aligning the primary lightbeam 22 with the secondary light beam 26 and aligning and focussing thenormally reflected beam 32 with the detecting system 30 (assumehereinafter that the same adjustments are made with respect to the rearimaging system 12). It is emphasized here again that the primary lightbeam 22, the secondary light beam, and the reflected beam 32 are eachrectangular in shape so that a "slice" from the top to the bottom of adocument travelling through track 14 is imaged. The basic concept foraligning the primary and secondary light beams 22 and 26 is best shownin FIG. 2 and the basic concept for aligning and focusing the reflectedbeam 32 with the detecting system 30 is best shown in FIG. 3.

In FIG. 2, the primary light beam 22 and secondary light beam 26approach the track 14 from points symmetrically offset from a linenormal to the track 14. Illuminating documents from symmetrically offsetangles allows imaging the document with some depth of field which wouldnot be achievable if the document were simply illuminated from one side.Imaging the documents using a system with depth of field is importantsince the distance between the front and back track walls 16 and 18,respectively, is considerably larger than the thickness of the documentbeing imaged, i.e., the distance between the track walls 16 and 18 ispreferably 80 mils while the thickness of a personal or cashiers checkis on the order of 3 to 5 mils. In addition, shadowing effects caused bywrinkled documents are minimized or avoided by illuminating the documentfrom opposite angles. In a preferred embodiment, the primary andsecondary light beams 22 and 26, respectively, are symmetrically offsetfrom the center line by approximately 30°. It has been found that makingthe offset angle wider, i.e., 45°, reduces the intensity of thereflected beam 32 to a point which is too low for adequate imaging bythe detection system 30. In order to align the center line 44 of primarylight beam 22 with the center line 46 of secondary light beam 26, thetrunnion mirror 28 is rotated. This adjustment treats the primary lightbeam 22 as fixed, with the secondary light beam 26 being adjusted withrespect to the primary light beam.

In FIG. 3, the path of the normally reflected beam 32 in the detectionsystem 30 is illustrated. The reflected beam 32 is shown in line formbut in practice is actually a rectangular beam of light. The beam isreflected from the document 50 (which will be a gauge during thealignment and focussing procedures described below) towards an inclinedmirror 52. The inclined mirror 52 directs the reflected beam 32 to atrunnion mirror 54 which reflects it towards a photodetector array 56.Before impinging on the photodetector array 56, the reflected beam 32passes through a photopic filter 58 and an imaging lens 60. The photopicfilter 58 is comprised of a plane glass plate with multiple layers ofdielectric film formed thereon. The dielectric film layers act asoptical coatings which are specially formulated to shape the spectralresponse of the system whereby the response of a human eye isapproximated. The photodetector array 56 may be a charge coupledphotodiode (CCPD) array which includes a plurality of rows ofphotodiodes arranged in the form of a rectangle. Alignment of thereflected beam 32 is achieved by rotating the trunnion mirror 54 untilthe reflected beam 32 impinges in the central part of the photodetectorarray 56. Focussing of the reflected beam 32 is achieved by moving theimaging lens 60 in a lateral direction toward and away from thephotodetector array 56.

The present invention is particularly concerned with the use of a set ofgauges to achieve proper alignment and focussing of the imaging systems10 and 12 (shown in FIG. 1). FIGS. 4a and 4b show front and cut-awayside views, respectively, of a depth gauge 70 which is used whenaligning the reflected beam 32 with the photodetector array 56 (shown inFIG. 3) and when aligning the central part 46 of secondary beam 26 withthe central part 44 of the primary beam 22 (shown in FIG. 2). FIGS. 5aand 5b show front and cut-away side views, respectively, of a focussinggauge 80 which is used when focussing the reflected beam onto thephotodetector array 56 (shown in FIG. 3). FIGS. 6a and 6b show front andcut-away side views of a "white reference" gauge 90 which is used tonormalize the output of the photodetector array 56. The cut-away sideviews of FIGS. 4b, 5b, and 6b are taken from lines 4a, 5a, and 6a, inFIGS. 4a, 5a, and 6a, respectively.

Referring now to FIGS. 4a and 4b, the depth gauge 70 is rectangular inshape and being removable, fits in the track 14 in front of the windowfor the imaging system 10 or 12. The depth gauge 70 is made from metal,plastic, or some other suitable material. The top and bottom ends, 71and 72, respectively, and left and right sides, 68 and 69, respectively,of the depth gauge 70 are thick enough to fit snugly within the track14, i.e., in a preferred embodiment they are approximately 80 mils thick(see thickness W₄, FIG. 4b between gauge front surface 79_(F) and rearsurface 79_(R)) which is equivalent to the distance between the frontand rear track walls, 16 and 18, respectively. The middle region of thedepth gauge 70 is machined to have a series of thicker 74 and thinner 75plateau areas which form stripes across the height of the depth gauge70. The thicker 74 and thinner 75 plateau areas have a reflectivecoating positioned thereon. The reflective coating may be in the form ofa white paint or chemical coating, a white plastic material affixed tothe gauge, or any other suitable coating. The reflective surface on thethicker 74 and thinner 75 plateau areas could also be provided byconstructing the gauge 70 from a white plastic material. The left andright sides 68 and 69 make the depth gauge 70 more rugged and helpresist warping and bending in the middle region. In addition, the leftand right sides 68 and 69 help prevent scraping of the reflectivecoatings on the thicker 74 and 75 thinner plateau areas when the depthgauge 70 is inserted or removed from the track.

The distance 76 between the center line 77 through the depth gauge 70 tothe thicker plateau areas 74 is equal to the distance 78 between thecenter line 77 through the depth gauge 70 to the thinner plateau areas75. In one embodiment, the center line 77 through the depth gauge 70 is40 mils from the back, flat surface of the depth gauge 70, the thickerplateau areas 74 are 60 mils thick, and the thinner plateau areas 75 are20 mils thick. It is contemplated that the thicker 74 and thinner 75plateau areas could be machined to graduated thicknesses wherein each ofthe thicker 74 plateaus has a different, graduated thickness and each ofthe thinner 75 plateaus has a correspondingly thinner, graduatedthickness.

Referring now to FIGS. 5a and 5b, the focussing gauge 80 is the samerectangular shape as the depth gauge 70 and is fabricated from the sameor similar materials. The top and bottom ends, 82 and 84, respectively,and the left and right sides, 81 and 83, respectively, of the focussinggauge 80 are sized to fit snugly within the track 14, i.e., 80 mils inthe preferred embodiment (see thickness W₅ between front and rearsurfaces 89_(F), 89_(R), FIG. 5b). The middle region 86 of the focussinggauge 80 is machined to a thickness equal to half the overall thicknessof the gauge 80, i.e., 40 mils. A series of black 87 and white 88stripes are positioned in the middle region 86 of the focussing gauge80. The black 87 and white 88 stripes may be photographically reproducedand attached to the middle region 86, painted in the middle region, orsupplied by some other suitable means.

Referring now to FIGS. 6a and 6b, the "white reference" gauge 90 is thesame rectangular shape as the depth gauge 70 and the focussing gauge 80and is fabricated from the same or similar materials. The top and bottomends, 92 and 94, respectively, and the left and right sides, 91 and 93,respectively, of the "white reference" gauge 90 are sized to fit snuglywithin the track 14, i.e., 80 mils in a preferred embodiment (seethickness W₆ between front and rear surfaces 99_(F), 99_(R), FIG. 6b).The middle region 96 is machined down to a level slightly less than theoverall thickness of the gauge 90, i.e., in a preferred embodiment it ismachined down 15 mils from the 80 mils thickness of the gauge 90 to athickness of 65 mils. The middle region 96 has a highly reflective whitecoating such as white paint, an attached white plastic material, or anyother suitable coating.

Referring back to FIGS. 1 through 3, alignment and focussing of theimaging system 10 or 12 is a three step procedure. In each step, thephotodetector array 56 is connected to an oscilloscope and the intensityof the light impinging on the individual photodetectors in the array isshown as a wave on the oscilloscope display. First, the depth gauge 70is placed in the track 14 in front of the window for the particularimaging system 10 or 12 being adjusted and the reflected beam 32 isaligned with the photodetector array 56. In the first step, only theprimary beam 22 or 22' is used to illuminate the depth gauge 70. Thesecondary beam 26 or 26' is eliminated simply by turning trunnion mirror28 or 28' out of the way. The trunnion mirror 54 is then rotated so asto reflect the reflected beam 32 towards the photodetector array 56.Alignment of the reflected beam 32 is achieved when it impinges on theactive part of photodetector array 56. Second, while the depth gauge 70is still in place, the trunnion mirror 28 is then rotated back and isused to reflect the secondary beam 26 or 26' towards the track 14.Alignment of the secondary beam 26 is achieved when the central portion46 of the secondary beam 26 coincides with the central portion 44 of theprimary beam 22. Third, the focussing gauge 80 replaces the depth gauge70 in the track 14 and the reflected beam 32 is focussed on thephotodetector array 56 by laterally sliding the imaging lens 60 towardsand away from the photodetector array 56. Focussing of the reflectedbeam 32 is achieved when a maximum intensity variation between the lightreflected from the white stripes 88 and the light reflected from theblack stripes 87 is achieved.

FIGS. 7a-7c and 8a-8c illustrate the alignment procedure for aligningthe reflected light beam 32 with the active part 57 of photodetectorarray 56. Comparison to FIGS. 2, 3, 4a and 4b may be helpful inexplaining the alignment procedure. Reflected light beam 32 representsthe light reflected normally from the surface of depth gauge 70positioned in the track 14 when it is illuminated with a rectangularlight beam (i.e., primary light beam 22) from a 30° angle. Secondarylight beam 26 has been eliminated as an illumination source by rotatingtrunnion mirror 28 out of the way. FIG. 7a shows that if the reflectedlight beam 32 intersects the trunnion mirror 54 at too wide an angle thereflected light beam 32 will impinge above and away from the active part57 of the photodetector array 56. FIG. 8a shows an oscilloscope display100 of the electrical signals 102 output from the photodetectors in thephotodetector array 56 when the trunnion mirror 54 is positioned asshown in FIG. 7a. FIGS. 7c and 8c shows a similar result to that shownin FIGS. 7a and 8a. In FIG. 7c, the reflected light beam 32 intersectsthe trunnion mirror 54 at too narrow of an angle and is directed belowthe active portion 57 of the photodetector array 56. FIG. 8c shows anoscilloscope display 100 of the electrical signals 104 output from thephotodetectors in the photodetector array 56 when the trunnion mirror 54is positioned as shown in FIG. 7a.

FIGS. 7b and 8b show the position of the trunnion mirror 54 and theelectrical signals 106 output on the oscilloscope display 100 when thereflected light beam 32 is properly aligned. FIG. 7b shows the reflectedlight beam 32 is directed by the trunnion mirror 54 towards the activepart 57 of the photodetector array 56. FIG. 8b shows the electricalsignals 106 on the oscilloscope 100 are much more intense compared withthe signals 102 and 104, indicated in FIGS. 8a and 8c, respectively,because the active part 57 of the photodetector array 56 directlyreceives more intense light. Even when the reflected light beam 32 isincident on the active part 57 of the photodetector array 56, the highintensity signals 106 may not be perfectly level. This is because theplateaus 74 and 75 on the depth gauge 70 may not be equally illuminatedat the particular part of the depth gauge 70 imaged onto thephotodetector array 56. Small adjustments of the trunnion mirror 54 canbe made to image the part of the depth gauge 70 which is illuminatedequally on plateaus 74 and 75. At this position of the trunnion mirror54, the light 32 imaged onto the photodetector array 56 will not only behighest but also most uniform. At this position of the trunnion mirror54, the light imaged onto the photodetector array 56 is coming from thepart of the track which is most uniformly illuminated by the primarybeam 22; i.e., the illumination of the document will be maximized, andas near as possible, the same, whether the document passes through nearthe front 16 of the track 14 or near the back 18 of the track 14.

FIGS. 9a-9c, 10a-10c and 11a-11c illustrate this principle in moredetail, with specific reference to the procedure for aligning thesecondary beam 26 with the primary beam 22. Comparison with FIGS. 2, 4a,and 4b may be helpful in explaining this alignment procedure. The depthgauge 70 is positioned in the track 14 and is illuminated by both theprimary and secondary light beams, 22 and 26, respectively, i.e., thetrunnion mirror 28 is rotated to reflect the secondary beam 26 towardsthe track 14. The objective is to align the central portion 46 of thesecondary beam 26 with the central portion 44 of the primary beam 22.FIGS. 9a-9c and 10a-10c respectively show top and front side views ofthe depth gauge 70 positioned in the track 14. FIGS. 9a-9c show arrows150 and 160 directed towards the track 14 which represent the centralportion 44 of the primary beam 22 and the central portion 46 of thesecondary beam 26, respectively, and an arrow 170 directed away from thetrack which represents the normally reflected light beam 32. FIGS. 9aand 9c show smaller arrows 172 and 174, respectively, which representlight reflected by only part of the plateaus 74 and 75 of the depthgauge 70. As discussed in connection with FIG. 4b, the depth gauge 70has machined therein a horizontally striped pattern of alternatingplateau regions 74 and 75 of thicker and thinner depths.

Referring now to FIGS. 2 and 10a-10c, the primary light beam 22 of FIG.2 is shown in FIGS. 10a-10c as a series of arrows 151-156 and thesecondary light beam 26 of FIG. 2 is shown in FIGS. 10a-10c as a seriesof arrows 161-166. As discussed above, both the primary and secondarylight beams, 22 and 26, respectively, are rectangular in shape. Avertical portion of the depth gauge 70 is exposed through the windowopening, defined by lines 200 and 202, of the front wall 16 of the track14. Each arrow 151-156 and 161-166 is representative of the centralregion 44 or 46, respectively, of the primary and secondary light beams22 and 26 at a particular height on the depth gauge 70. FIG. 10b showsthe light beams represented by arrows 161-166 perfectly aligned with thelight beams represented by arrows 151-156. The condition shown in FIG.10b is ideal and represents a situation when the secondary beam 26 is inperfect alignment with the primary beam 22. When comparing FIGS. 9b and10b, a strong, normally reflected light beam 170 will be detected by thephotodetector array 56 (shown in FIG. 3) because the point at which thearrows 151-156 and 161-166 strike each of the plateaus 74 and 75 isaligned with the active portion 57 of the photodetector array 56. BothFIGS. 10a and 10c show the light beams represented by arrows 161-166 outof alignment with the light beams represented by arrows 151-156. Theconditions shown in FIGS. 10a and 10c represent situations where thesecondary beam 26 is directed too far to the left or too far to theright, respectively, and is out of alignment with the primary beam 22.

Referring to FIG. 10a, the light beams represented by arrows 161, 163,and 165 are directed to points toward the left 200 of the window200-202, while the light beams represented by arrows 162, 164, and 166are directed to points nearer the center of the window 200-202. Thecondition illustrated by FIG. 10a occurs because the thicker plateaus 74are physically closer to the window 200-202 opening than the thinnerplateaus 75. Comparing FIG. 10a with FIG. 9a, it is apparent that somelight 172 will be reflected back through the window 200-202 from thethicker plateaus 74, i.e., light from the light beams represented byarrows 162, 164, and 166. The light from light beams 161, 163, 165,which are directed towards the thinner plateaus 75, will be reflectedback through the window 200-202; however, the light will be shifted tothe left and not aligned with the active area on the photodetector array56. The photodetector array 56 (shown in FIG. 3) detects both the light172 reflected from light beams 162, 164, and 166 and the light 170reflected from the primary beam 22.

Referring to FIG. 10c, the opposite condition of that shown in FIG. 10ais illustrated. In FIG. 10c, the light beams represented by arrows 162,164, and 166 are directed to points toward the right 202 of window200-202, while the light beams represented by arrows 161, 163, and 165are directed to points nearer the center of the window 200-202. Like thecondition illustrated in FIG. 10a, the condition illustrated in FIG. 10coccurs because of the differing thicknesses of the plateau areas 74 and75. Comparing FIG. 10c with FIG. 9c, it is apparent that some light 174will be reflected back through the window 200-202 from the thinnerplateaus 75, i.e., light from the light beams represented by arrows 161,163, and 165. Conversely, the light from light beams 162, 164, 166,which are directed towards the thicker plateaus 74, will be reflectedback through the window 200-202, but it will be shifted to the right andnot aligned with the active area on the photodetector array 56. Thephotodetector array 56 (shown in FIG. 3) detects both the light 174reflected from light beams 161, 163, and 165 and the light 170 reflectedfrom the primary beam 22.

FIGS. 11a-11c illustrate the electrical signal output from thephotodetector array 56 (shown in FIG. 3) on an oscilloscope display 210when the secondary beam 26 is being aligned with the primary beam 22.Comparing FIGS. 9a, 10a, and 11a, it is apparent that the intensity ofthe light reflected from the thicker plateau areas 74 of the depth gauge70 is far greater than the intensity of the light reflected from thethinner plateau areas 75. This is because the thicker plateau areas 74are reflecting some of the light 172 from the secondary beam 26(represented by arrows 162, 164, and 166) in addition to the reflectedlight 170 from the primary beam 22, while the thinner plateau areas 75do not reflect the secondary light 26 back to the active area of thephotodetector array 56. FIG. 11a illustrates that when the secondarybeam 26 is directed too far to the left, a toothed output 212 will beshown on the oscilloscope display 210 where peaks 214 represent theadditional light 172 reflected from the thicker plateau areas 74 and thevalleys 216 represent the lack of reflected light from the thinnerplateau areas 75. Likewise, when comparing FIGS. 9c, 10c, and 11c it isapparent that the intensity of the light reflected from the thinnerplateau areas 75 of the depth gauge 70 is far greater than the intensityof the light reflected from the thicker plateau areas 74. This isbecause the thinner plateau areas 75 are reflecting some of the light174 from the secondary beam 26 (represented by arrows 161, 163, and 165)in addition to the reflected light 170 from the primary beam 22, whilethe thicker plateau areas 74 do not reflect the secondary light 26 backto the active area of the photodetector array 56. FIG. 11c illustratesthat when the secondary beam 26 is directed too far to the right, atoothed output 220 will be shown on the oscilloscope display 210 wherepeaks 222 represent the additional light 174 reflected from the thinnerplateau areas 75 and the valleys 224 represent the lack of reflectedlight from the thicker plateau areas 75.

The perfect alignment condition is illustrated with FIGS. 9b, 10b, and11b. In FIG. 11b, the oscilloscope display 210 shows a nearly flat(across the top) output 230 from the photodetector array 56 (shown inFIG. 3) when the light beams represented by arrows 161-166 are alignedwith the light beams represented by arrows 151-156 as shown in FIG. 10b.When the primary and secondary light beams, 22 and 26, respectively, arealigned, the reflected beam 32 (shown in FIGS. 2 and 3 and discussed aslight beam 170 in FIG. 9b) has its maximum intensity since light fromtwo sources is being reflected.

FIGS. 12a-12c and 13a-13c illustrate the procedure for focussing thereflected light beam 32 on the photodetector array 56. Reference back toFIGS. 2, 3, 5a, and 5b will aid in understanding this explanation. Thefocussing step is performed after alignment of the reflected beam 32relative to the photodetector array 56 has occurred (discussed above inconjunction with FIGS. 7a-7c and 8a-8c),i.e., the focussing step may beperformed before or after aligning the primary and secondary lightbeams, 22 and 26, respectively (discussed above in conjunction withFIGS. 9a-9c, 10a-10c, and 11a-11c). It is also possible to use aniterative procedure in which the alignment steps and the focussing stepsare repeated until a satisfactory optimum is obtained.

To perform the focussing step, the focussing gauge 80 (shown in FIGS. 5aand 5b) replaces the depth gauge 70 in the track 14. Focussing isachieved by moving imaging lens 60 laterally toward and away from thephotodetector array while observing the measured intensity of thereflected light 32 from the black 87 and white 88 striped pattern on thefocussing gauge 80 on the oscilloscope display 300. On the oscilloscopedisplay 300, the peaks 302 represent light reflected from a white stripe88 and the valleys 304 represent light reflected from a black stripe 87.Focussing is achieved when a maximum difference exists between the peaks302 and valleys 304. FIGS. 12a and 13a show that when the imaging lens60 is too far from the photodetector array 56, the reflected beam 32 isout of focus (i.e., focussed to a point in front of the photodetectorarray) and the intensity of the peaks 302 is not very much greater thanthe valleys 304. FIGS. 12c and 13c show that when the imaging lens 60 istoo close to the photodetector array 56, the reflected beam 32 is out offocus (i.e., focussed to a point behind the photodetector array) and theintensity of the peaks 302 is not very much greater than the valleys304. FIGS. 12b and 13b illustrate the condition when the reflected beam32 is focussed properly. When the imaging lens 60 is positioned theright distance away from the photodetector array, the reflected beam 32will be focussed directly on the photodetector array 56. FIG. 13b showsthat when the reflected beam 32 is precisely focussed, the oscilloscopedisplay 300 shows a maximum difference between the peak 302 and valley304 intensities. The frequency of the peaks 302 and the valleys 304 maynot correspond to the frequency of the black 87 and white 88 stripes onthe focussing gauge 80. This is a well known effect in imaging systemswhich use an array of discrete detectors to image repetitive patterns,and is referred to in the literature as "aliasing", "beat frequency", or"Moire effect frequency". The principle is still valid that thedifference between the peaks and the valleys should be maximized for thebest image focus.

After the imaging system 10 or 12 (shown in FIG. 1) has been aligned andfocussed, the "white reference" gauge (shown in FIGS. 6a and 6b) isinserted in the track 14 (shown in FIG. 2) so that the output from thephotodetector array 56 (shown in FIG. 3) can be normalized.Normalization is a well known practice where the intensity output from adetector is compared to a reference amount. Normalization recognizesthat the signal from a uniform document will not be the same from eachelement of the detector array. This lack of signal uniformity may becaused by the illumination of the document not being perfectly uniform,the photodetector elements in the photodetector array having differentsensitivities, the signal processing electronics being different fordifferent parts of the photodetector array, or the imaging lensproducing some shading of the image. The normalization process measuresfor each photodetector information channel in the array, the signal froma "black" document or test piece and the signal from a "white" referencedocument or test piece. Signals measured from the real documents beingprocessed during the operation of the document processor are comparedwith the reference measurements. Typically, the black reference signalwill be subtracted from the measured signal and the result will beexpressed as a fraction of the difference between the white and blackreference signals. In the present application, the intensity of lightreceived by the photodetector array 56 when the "white reference" gauge90 is in the track 14 will be used as the reference amount. The purposeof normalization is to improve the response of the imaging system 10 or12 to writing on a document relative to the background, i.e., thebackground will be subtracted out using the reference response. FIG. 14ashows that an optimum response curve 420 will rise above a thresholdvalue 410 on the oscilloscope display 400, while FIG. 14b shows that aninferior response curve 430 will not rise above the threshold value 410.

While the invention has been described in terms of a preferredembodiment where a set of three track gauges are used to align, focusand normalize an imaging system in a high speed document handlingdevice, those skilled in the art will recognize that the invention canbe practiced with modification within the spirit and scope of theappended claims.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. A depth gauge used foraligning an imaging system in a high speed document processor,comprising:a gauge having height and width dimensions larger than arectangular window in a track of said high speed document processor,said rectangular window allowing said imaging system to image itemswithin said track; a means for holding said gauge said track; and atleast two plateaus formed in a front portion of said gauge, a firstplateau being positioned a first distance from a back portion of saidgauge which is a fixed amount less than a second distance between saidback portion of said gauge and a plane running through a vertical axisof said gauge, a second plateau being positioned a third distance fromsaid back portion of said gauge which is said fixed amount more thansaid second distance.
 2. A depth gauge as recited in claim 1 whereinsaid gauge is rectangular in shape.
 3. A depth gauge as recited in claim1 wherein said means for holding said gauge snugly within said trackcomprises a portion of said gauge having a thickness approximatelyequivalent to a fourth distance between front and rear walls which formsaid track.
 4. A depth gauge as recited in claim 3 wherein said portionof said gauge having said thickness approximately equivalent to saidfourth distance encircles said first and second plateaus.
 5. A depthgauge as recited in claim 3 wherein said second distance between saidback portion of said gauge and said plane running through said verticalaxis of said gauge is approximately half said fourth distance betweensaid front and rear walls which form said track.
 6. A depth gauge asrecited in claim 1 further comprising a white reflective coatingpositioned on said first and second plateaus.
 7. A depth gauge used foraligning an imaging system in a high speed document processor,comprising:a gauge having height and width dimensions larger than arectangular window in a track of said high speed document processor,said rectangular window allowing said imaging system to image itemswithin said track; a means for holding said gauge snugly within saidtrack; and at least two sets of plateaus formed in a front portion ofsaid gauge, a first set of plateaus being positioned a first distancefrom a back portion of said gauge which is a fixed amount less than asecond distance between said back portion of said gauge and a planerunning through a vertical axis of said gauge, a second set of plateausbeing positioned a third distance from said back portion of said gaugewhich is said fixed amount more than said second distance.
 8. A depthgauge as recited in claim 7 wherein said first and second sets ofplateaus are arranged in a horizontal stripe pattern extending up saidheight dimension of said gauge, each of the plateaus in said first setof plateaus being adjacent a plateau of said second set of plateaus insaid horizontal stripe pattern.
 9. A depth gauge as recited in claim 7wherein said gauge is rectangular in shape.
 10. A depth gauge as recitedin claim 8 wherein said means for holding said gauge snugly within saidtrack comprises a portion of said gauge having a thickness approximatelyequivalent to a fourth distance between front and rear walls which formsaid track.
 11. A depth gauge as recited in claim 10 wherein saidportion of said gauge having said thickness approximately equivalent tosaid fourth distance encircles said horizontal stripe pattern.
 12. Adepth gauge as recited in claim 10 wherein said second distance betweensaid back portion of said gauge and said plane running through saidvertical axis of said gauge is approximately half said fourth distancebetween said front and rear walls which form said track.
 13. A depthgauge as recited in claim 7 further comprising a white reflectivecoating positioned on each plateau in said first and second sets ofplateaus.
 14. A focussing gauge used for focussing with lens means of animagining system on a linear sensor having a prescribed number of likecells, in a high speed document processor, this gauge comprising:framemeans of prescribed width surrounding a reference zone with height andwidth dimensions larger than a rectangular window in a track of saidhigh speed document processor, said rectangular window allowing saidimaging system to image items within said track; said frame means beingdimensioned to hold said gauge snugly within said track; a plateauformed in a front portion of said gauge, said plateau being positioned afirst distance from the back of said gauge which is a fixed amount lessthan the width of said track; and a black and white horizontal stripepattern positioned on said plateau, said black stripes having apredetermined width related to the size of said sensor cells, wherebyprecise focussing with said lens means can yield a "Moire" intensitypattern.
 15. A focussing gauge as recited in claim 14 wherein said gaugeis rectangular in shape.
 16. A focussing gauge as recited in claim 14wherein said means for holding said gauge snugly within said trackcomprises a portion of said gauge having a thickness approximatelyequivalent to said second distance between front and rear walls whichform said track.
 17. A focussing gauge as recited in claim 16 whereinsaid portion of said gauge having said thickness approximatelyequivalent to said second distance encircles said plateau.
 18. Afocussing gauge as recited in claim 14 wherein said fixed amount lessthan said second distance is approximately half of said second distance.19. A focussing gauge as recited in claim 14 wherein said black andwhite horizontal stripe pattern is painted on said plateau.
 20. Afocussing gauge as recited in claim 14 wherein said black and whitehorizontal stripe pattern is photographically produced and attached tosaid plateau.
 21. A white reference normalization gauge used fornormalizing with lens means of an imaging system on a linear sensorhaving a prescribed number of like cells, in a high speed documentprocessor, this gauge comprising:frame means of prescribed widthsurrounding a reference zone with height and width dimensions largerthan a rectangular window in a track of said high speed documentprocessor, said rectangular window allowing said imaging system to imageitems within said track; said frame means being dimensioned to hold saidgauge snugly within said track; a plateau formed in a front portion ofsaid gauge, said plateau being positioned a first distance from a backportion of said gauge which is a fixed amount less than the width ofsaid track; and a white reflective coating formed on said plateau.
 22. Awhite reference gauge as recited in claim 21 wherein said gauge isrectangular in shape.
 23. A white reference gauge as recited in claim 21wherein said means for holding said gauge snugly within said trackcomprises a portion of said gauge having a thickness approximatelyequivalent to said second distance between front and rear walls whichform said track.
 24. A white reference gauge as recited in claim 23wherein said portion of said gauge having said thickness approximatelyequivalent to said second distance encircles said plateau.
 25. A depthgauge for aligning an oblique two-beam illumination arrangement as ahigh speed document processor for transporting documents along a trackmeans of prescribed constant track width, many times the width of saiddocuments, said track means including at least one rectangular windowmeans cut into said track means and defining an illumination site ofprescribed uniform height and width, said documents being prone to passsaid window means at varying positions across said track width, saidgauge comprising:frame means surrounding a relatively rectangulargauge-locus which is at least as high and as wide as said illuminationsite, said frame means being adapted for selective removable insertioninto said track means at a said window means so as to firmly positionsaid gauge-locus at a said site, with the height and width of thegauge-locus spanning the height and width of said window means; saidgauge-locus being characterized by at least two sets of reflectingplateaus all disposed on a common base plane, each plateau set beingrelatively uniformly dispersed across at least one dimension of saidlocus and being formed to present a number of reflecting surfaces havinga prescribed constant height above said base plane, whereby the beamsmay be aligned at various document locations across the track width. 26.The gauge of claim 25 wherein one said number of reflecting surfaces hasa uniform height above said base plane which is somewhat more than halfthe width of said track means and wherein another said number ofreflecting surfaces has a uniform height which is somewhat less thanhalf the width of said track means, whereby to simulate two differentdocument positions across the width of said track means.
 27. The gaugeof claim 25 wherein all said reflecting surfaces are finished and coatedto exhibit high reflectance to spectral light beams incident obliquethereto.